Submissions are evaluated on Root-Mean-Squared-Error (RMSE) between the logarithm of the predicted value and the logarithm of the observed sales price. (Taking logs means that errors in predicting expensive houses and cheap houses will affect the result equally.)

RMSE = sqrt( mean( ( log(y) – log(haty) )^2 ) )

During the competition, the leaderboard displays your partial score, which is the RMSE for 735 (random) houses of the test set.
At the end of the contest, the leaderboard will display the final score, which is the RMSE for the remaining 735 houses of the test set. The final score will determine the final winner. This method prevents users from overfitting to the leaderboard.